For example, Patent Document 1 (Japanese Patent No. 5064824) discloses a HEMT. The HEMT has a heterojunction structure which is formed by stacking, on a substrate, a low-temperature buffer layer formed of GaN, a buffer layer formed of GaN, an electron transit layer formed of GaN and an electron supply layer formed of AlGaN in this order. The HEMT has a source electrode, a gate electrode and a drain electrode on the electron supply layer.
In the HEMT, the electron supply layer has a band-gap energy larger than the electron transit layer, and a two-dimensional electron gas layer is formed under a heterojunction interface of the two layers. The two-dimensional electron gas layer is utilized as a carrier. Specifically, when the source electrode and the drain electrode are operated, electrons supplied to the electron transit layer travel at a high speed in the two-dimensional electron gas layer and are moved to the drain electrode. Here, a voltage applied to the gate electrode is controlled to change the thickness of a depletion layer under the gate electrode, and it is thus possible to control the electrons moved from the source electrode to the drain electrode, that is, a drain current.